DECEMBER
2006

 

 

 

By
Tom Turpin
 
Professor of
Entomology
Purdue University

 

 

 

 

 

12-14-06

Ectoparasites Can Be Lousy Subject for Some People

According to the ancient Greeks, a parasite was someone who ate at the table of another without paying. At least a parasite didn't pay for food with money. Such individuals earned their meal by flattering their host. Obviously the old saying "flattery will get you nowhere" was not entirely true in ancient Greece!

Today, as in the ancient world, the word parasite is generally not used as a term of endearment. People who reap more benefits, from government or otherwise, than some think they deserve are often called parasites. Sycophants, people who hang around the rich and famous, are defined as a type of human parasite. Biologically, parasites are plants or animals that live in, on or with another living organism. Generally, but not always, the parasite harms the host. Consequently, the concept of parasitism, even in biology, is rather negative.

Some parasites are called ectoparasites. That is because these organisms live on the outside of the host. Endoparasites, on the other hand, live inside the host. There are a number of insect species that are ectoparasites. Most of the insect ectoparasites are types of fleas and lice. And, since fleas and lice use humans as a host, for most of us, that literally is where the rub comes in!

Both fleas and lice in the adult stage feed on human blood. However, there is a major difference in the two groups of insects as to their feeding habits. In general, fleas are not very choosy about the specific host on which they feed. There are so-called cat fleas, dog fleas and the appropriately named human flea, Pulex irritans. But, in spite of their namesake animal, almost all fleas will feed on other host species. The cat flea is the most common species found feeding on dogs. Dog fleas, as the name suggests, will feed on dogs, humans, cats, and most other mammals as well.

Lice, on the other hand, are more finicky feeders. These insects normally dine on only a single species of host. Chicken lice feed on chickens, cattle lice on cattle and hog lice on pigs. Even though hog lice will feed only on hogs, these insects can sometimes be found crawling on other animals, such as hog farmers. And that behavior leads to the incorrect notion that the insect was probably feeding.

We humans, as it turns out, have two species of lice that call us home and dinner. Or maybe three species. In the past, some scientists have looked at human lice and divided these insects into three groups, based on the area of the body where they were found. Consequently, the names head lice, body lice and pubic lice were used for these insects that were found on our bodies.

Based on appearance, human lice are clearly of two different kinds. Generally, these two types have been called body lice and pubic lice. Pubic lice are sometimes called crab lice because of their crab-like body shape. As it turns out, the so-called body and head lice are the same species. They just show up feeding in different locations on the body.

Both types of human lice have claws that allow them to clasp human hair and not fall off their host. For that reason, lice populations in children are almost always found on the head. That is the only place on a child where lice can get a grip!

Unlike fleas, lice spend their entire life on their host animal. Lice eggs are glued to hair and are called nits. Many animals, including humans in some modern societies and almost all societies in bygone days, searched for and removed the nits in a process known as nit picking. We mostly are reminded of the process today when someone is called a "nit picker" because they appear to be dealing with minute, unimportant details.

So with their intimate association to a host, how do lice infestations get started on new hosts? The answer is through close personal contact. From a human perspective, "If you don't want lice, stay away from lousy people!" Good advice for more reasons than just dealing with ectoparasites.

 

Writer: Tom Turpin
Editor: Olivia Maddox